Greater shear strength for fiber reinforced composites is achieved by improving the fiber to matrix bond. A strong bond reduces the chance of failure at the interface. The strength of bonding may be measured in accordance with ASTM D 2344-76 and is referred to as interlaminar shear strength (ILSS). U.S. Pat. No. 3,723,607 deals with modification of the surface characteristics of amorphous or graphitic carbon fiber to provide enhanced adhesion between the fibrous material and a resinous matrix material. In this prior art patent, the fibrous material is first treated with an inert gaseous atmosphere at a temperature of about 900.degree. to 1400.degree. C. for at least 30 seconds and then, according to the examples given, is immediately placed in a heat treatment zone containing ozone. The patent teaches that the ozone treatment should be carried out in an atmosphere containing about 1500 to 110,000 parts per million of ozone and at a temperature of about 75.degree. to 175.degree. C. Failure to carry out either the high temperature inert gas treatment or the ozone treatment is taught to give inferior adhesion results although such results are said to be better than that obtained if neither treatment is applied. Use of temperatures below 75.degree. C. for the ozone treatment is said in the patent to slow down the modification to an inordinate extent. The patent does not disclose the treatment of pitch-based carbon fiber. Modification of the surface of pitch-based carbon fiber to increase adhesion to matrix polymer of composites is a worthwhile objective.